1. Field of the Invention
Embodiments of the present invention relate to an isolator and method of manufacturing an isolator.
2. Description of the Related Art
Various types of apparatuses, such as devices used for industrial or medical purposes, often require electrical insulation. For example, an electronic device controlled by high voltage is equipped with a signal isolator (isolator) so that when a signal received by the electronic device is conveyed to an external device or when the user directly manipulates the operation panel, the user is not subjected to electrical shock or other severe bodily harm. When the electric potential between electronic devices or circuit blocks is large, the isolator has a function of transferring (transmitting) a signal from a low voltage unit to a high voltage unit or from a high voltage unit to a low voltage unit, while current is interrupted (insulated).
For example, the high voltage unit and the low voltage unit are electrically isolated from each other by the isolator to prevent large current from flowing between the high voltage unit and the low voltage unit. Even when the electric potential between electronic devices or between circuit blocks is small and, for example, an analog circuit and a digital circuit are connected at a common reference potential, the analog circuit may be affected by digital noise. Therefore, by completely separating the reference potentials of the analog circuit and the digital circuit electrically by an isolator, digital noise may be prevented from being introduced at the analog circuit, enabling performance of the system to be improved.
Conventionally, signal isolating systems using a photocoupler that utilizes light as a signal transfer means are known as being among the most versatile isolators. The photocoupler has a structure incorporating a photodiode and a phototransistor in a single package, where light-dark changes in the light of the photodiode correspond to an input signal and are converted to voltage by the photo transistor to perform the electrically isolated signal transfer. Photocouplers have the advantages of a simple package configuration and high electrical isolation performance. On the other hand, photocouplers are devices that utilize light and therefore, cannot be fabricated by a general complementary metal oxide semiconductor (CMOS) technique and are mounted as a discrete component.
Further, the luminous efficiency of photodiodes configuring the photocoupler essentially degrades temporally consequent to operating conditions such as operating temperature and forward current. Therefore, when the life expectancy of a system equipped with the photocoupler is important, the setting of conditions such as operating temperature and forward current must be carefully considered. The photocoupler has a slow response speed and requires periods on the order of microseconds for signal transfer. As a result, for example, for an inverter drive system equipped with a photocoupler, the dead time of a device configured by the inverter must be established on the order of microseconds and therefore, increases in speed are not possible.
Coupling capacitors that utilize electric field changes consequent to capacitive coupling at the signal transfer means are known to be among the next highly versatile isolators after photocouplers. Since electric field changes are utilized for transferring a signal, the coupling capacitor interrupts direct current (DC) signals from a transmission circuit and transfers only alternating current (AC) signals to a reception circuit. Therefore, coupling capacitors are useful for separating the direct current voltage settings of a circuit network between the transmission circuit and the reception circuit. Coupling capacitors have the advantages of high insulation resistance and low power consumption. However, coupling capacitors have a problem of being easily affected by noise and external electric fields.